In recent years, for further improvement in the performance of semiconductor devices such as speeding up and an increase in the capacity, research and development of a three-dimensional integrated circuit having at least two layers of semiconductor device chips laminated to constitute a three-dimensional (3D) lamination, is in progress in addition to refinement of transistors and wiring.
A three-dimensional integrated circuit has a structure such that semiconductor device chips are connected e.g. by electric signal terminals such as solder bumps and are bonded by a filling interlayer formed by being filled with an interlayer filler.
Specifically, such a process is proposed (for example, Non-Patent Document 1) that a coating liquid of a composition (a composition for forming a filling interlayer) is applied to a wafer to form a thin film, B-stage formation (cure for B-state) is carried out by heating, and then chips are cut out by dicing, a procedure of laminating a plurality of such chips and temporary bonding the chips by pressure heating is repeatedly carried out, and finally main bonding (solder bonding) is carried out under pressure heating conditions.
For practical use of such a three-dimensional integrated circuit device, various problems have been pointed out, and one of them is a problem of dissipation of heat generated from a device such as a transistor or wiring. This problem results from a commonly very low coefficient of thermal conductivity of a composition to be used for lamination of semiconductor device chips as compared with metals and ceramics, and there are concerns about a decrease in the performance due to accumulation of heat in laminated device chips.
As one means to solve the problem, an increase in the coefficient of thermal conductivity of the composition for forming a filling interlayer may be mentioned. Specifically, a highly thermally conductive epoxy resin is used as a thermosetting resin constituting an adhesive component of the composition, or such a highly thermally conductive resin is combined with a highly thermally conductive inorganic filler, to make the composition be highly thermally conductive.
For example, Patent Document 1 discloses a composition having spherical boron nitride agglomerates blended as a filler.
Further, a composition has been required to have not only improved thermal conductivity but also compatibility to the 3D lamination process and capability in being formed into a thin film, and further connection properties to electric signal terminals between semiconductor device chips, and further technical development has been required.
That is, in a conventional process of mounting semiconductor device chips on an interposer or the like, first, electric signal terminals such as solder bumps on the semiconductor device chip side are subjected to activation treatment by a flux, and then bonded to a substrate having lands (electric connection electrode), and the space between the substrates is filled with a liquid resin or an underfill material having an inorganic filler added to a liquid resin, which is cured, whereby the bonding is carried out.
On that occasion, the flux is required to have properties to remove the surface oxide film on the metal electric signal terminals such as solder bumps and the lands, and to improve wettability, and further, an activation treatment function such as prevention of reoxidation on the metal terminal surface.
On the other hand, in the 3D lamination process of semiconductor device chips, if activation treatment of the electric signal terminals such as solder bumps using a flux is carried out first, a flux layer having low thermal conductivity is formed on the surface of the terminals, and there are concerns about inhibition of thermal conductivity between the laminated substrates by composition, or deterioration by corrosion of connection terminals by remaining flux components.
Accordingly, a flux which can be directly mixed with a composition having high thermal conductivity and which is less likely to corrode metal terminals has been desired.
As such a flux, not only an inorganic metal salt containing halogen excellent in capability in solving the metal oxide film of the electric signal terminals, but also an organic acid or an organic acid salt, an organic halogen compound or an amine, rosin or its constituent, is used alone or in combination of two or more of them (for example, Non-Patent Document 2).
As described above, a conventional interlayer filler composition usually contains a thermosetting resin as an adhesive component, an inorganic filler and a flux, and this composition is usually applied to a semiconductor substrate in the form of a coating liquid as dispersed or dissolved in a proper organic solvent thereby having an appropriate viscosity.
In recent years, particularly in the electric/electronic fields, heat generation due to high densification of integrated circuits is significantly problematic, and how to dissipate heat is an urgent problem.
In a conventional composition, boron nitride (BN) has been used as a filler.
Boron nitride (BN) is an insulating ceramic, and various crystal forms such as c-BN having a diamond structure, h-BN (hexagonal boron nitride) having a graphite structure, α-BN having a turbostratic structure and β-BN have been known.
Among them, h-BN having a graphite structure has characteristics such that it has the same layer structure as graphite, it is relatively easily prepared and it is excellent in the thermal conductivity, solid lubricity, chemical stability and heat resistance, and is widely used in the electric/electronic material fields.
Further, h-BN attracts attention, by making use of its characteristics such that it has high thermal conductivity although it is insulating, as a thermally conductive filler for a heat dissipation member for such applications, and is considered to be capable of forming a filling interlayer excellent in the heat dissipation properties.
In addition to the above Patent Document 1, techniques using a boron nitride powder have been known, and as such boron nitride, for example, a boron nitride powder having specific particle size and particle size distribution has been known (for example, Patent Document 3). Further, aside from this, a technique (for example, Patent Document 4) has been known to use a mixture of two types of boron nitrides differing in the particle properties such as the surface area, the particle size and the tap density.
Further, a h-BN powder is known as a material excellent in the thermal conductivity, and a technique has been known (for example, Patent Document 5) in which a crude hexagonal boron nitride powder is washed with water and subjected to heat treatment in a stream of an inert gas at from 1,500 to 1,800° C. to prepare highly crystalline boron nitride. By this technique, the crystallite size (La) of the 100 plane of crystallites of boron nitride is developed.
Further, Patent Document 6 discloses that a hexagonal boron nitride powder having a large particle size and having excellent lubricity can be obtained by applying a specific treatment to a crude hexagonal boron nitride powder.
Further, Patent Document 7 discloses a hexagonal boron nitride powder obtained by mixing a compound containing lanthanum as the main component with a crude hexagonal boron nitride powder and subjected the mixture to heat treatment in a non-oxidizing gas atmosphere within a specific temperature range to develop the crystallite size (Lc) of the 002 plane, and discloses that this hexagonal boron nitride powder is excellent in the dispersibility and has high crystallinity.
Further, with respect to hexagonal boron nitride, Patent Document 8 discloses that a crude hexagonal boron nitride powder is aged in the atmospheric pressure at 60° C. or below for at least one week, and then subjected to specific baking, whereby a powder having a large particle size and high crystallinity can be obtained.
Patent Document 9 discloses a hexagonal boron nitride powder developed to have an average particle size of 10 μm or larger by aging a crude hexagonal boron nitride powder in the atmospheric pressure at 60° C. or below for at least one week, washing it and subjecting it to specific baking treatment.
However, h-BN is in the form of plate-shaped particles, and has high thermal conductivity (usually, a coefficient of thermal conductivity of about 250 W/mK) in its plane direction (the C-plane direction or the (002) plane direction) but has only low thermal conductivity (usually, a coefficient of thermal conductivity of from about 2 to 3 W/mK) in the thickness direction (the C-axis direction). Thus, if this is blended with a resin to prepare a coating liquid of a composition, which is applied to a substrate surface to form a coating film, and the coating film is heated for the B-stage formation, and further temporally bonding and main bonding are carried out to produce a three-dimensional integrated circuit, in the produced three-dimensional integrated circuit, the plate-shaped BN particles are aligned in the plane direction of the coating film, and the formed filling interlayer is excellent in the coefficient of thermal conductivity in the layer plane direction but has only a low coefficient in the thickness direction.
Heretofore, in order to improve the anisotropy of the thermal conductivity of the BN particles, a h-BN powder not having a scaly shape, which is less likely to be aligned as above even when blended with a resin, has been studied. Such a h-BN powder may, for example, be h-BN particles granulated by spray drying, or h-BN particles produced by sintering h-BN and grinding the sintered h-BN (for example, Patent Documents 2 and 10).
Further, as other agglomerated particles, BN particles in the form of a pine cone having h-BN particles prepared from a mixture of boric acid and melamine agglomerated without being aligned, have been proposed (for example, Patent Document 11).